The present invention relates to a device and a method for fracturing a geological hydrocarbon reservoir, as well as a method of production of hydrocarbons.
In the production of hydrocarbons, the permeability and/or the porosity of the material constituting the reservoir have an influence on the production of hydrocarbons, in particular on the rate of production and thus the profitability. This is in particular what is referred to in the article “Porosity and permeability of Eastern Devonian Shale gas” by Soeder, D. J., published in SPE Formation Evaluation, 1988, Vol. 3, No. 1, pp. 116-124, which describes the investigation of eight samples of Devonian shale gas, originating from the Appalachians. In particular, this article explains that the production of this shale gas presents the difficulty that the reservoir (i.e. the material constituting the reservoir) has low permeability.
Thus, various techniques exist for facilitating the rate of production of hydrocarbons, in particular from a reservoir of low permeability and of low porosity. These techniques consist of fracturing the reservoir statically or dynamically.
Static fracturing is a targeted dislocation of the reservoir, by injecting a fluid under very high pressure to crack the rock. Cracking is effected by a mechanical “stress” originating from hydraulic pressure obtained by means of a fluid injected under high pressure from a well drilled from the surface. It is also called “hydrofracturing” or “hydrosiliceous fracturing” (or else “frac jobs”, or more generally “fracking”, or “massive hydraulic fracturing”). Document US 2009/044945 A1 in particular presents a method of static fracturing as described above.
Static fracturing has the drawback that the fracturing of the reservoir is generally unidirectional. Thus, only the hydrocarbon present in the portion of the reservoir around a deep but highly localized crack is produced more quickly.
To obtain more diffuse fracturing, dynamic fracturing, or electrical fracturing, has been introduced. Electrical fracturing consists of generating an electric arc in a well drilled in the reservoir (typically the production well). The electric arc induces a pressure wave which damages the reservoir in all directions around the wave and thus increases its permeability.
Several documents discuss electrical fracturing. For example, document U.S. Pat. No. 4,074,758 presents a method consisting of generating an electro-hydraulic shock wave in a liquid in the wellbore to improve petroleum recovery. Document U.S. Pat. No. 4,164,978 suggests following the shock wave with an ultrasonic wave. Document U.S. Pat. No. 5,106,164 also describes a method of generating a plasma blast and thus fracturing a rock, but in the case of a borehole of small depth, for a mining application and not for production of hydrocarbons. Documents U.S. Pat. No. 4,651,311 and U.S. Pat. No. 4,706,228 present a device for generating an electric discharge with electrodes in a chamber containing an electrolyte, in which the electrodes are not subject to erosion by the plasma of the discharge. Document WO 2009/073475 describes a method of generating an acoustic wave in a fluid medium present in a well with a device comprising two electrodes between an upper packer and a lower packer defining a confined space. According to this document, the acoustic wave is maintained in a non-“shock wave” state in order to improve the damage, without however explaining the differences between “ordinary” acoustic wave and “shock” wave.
None of these documents produces entirely satisfactory fracturing of the reservoir. There is therefore a need for improved fracturing of a hydrocarbon reservoir.
For this, a device is proposed for fracturing a geological hydrocarbon reservoir, in which the device comprises two packers that between them define a confined space in a well drilled in the reservoir; a pump for increasing the pressure of a fluid in the confined space; an apparatus for heating the fluid; at least one pair of two electrodes arranged in the confined space; and an electric circuit for generating an electric arc between the two electrodes, the circuit comprising at least one voltage source connected to the electrodes and an inductance between the voltage source and one of the two electrodes. According to examples, the device can comprise one or more of the following features:                the inductance is an adjustable inductance coil, preferably between 1 μH and 100 mH, more preferably between 10 μH and 1 mH;        the distance between the electrodes is adjustable, preferably between 0.2 and 5 cm, more preferably between 1 and 3 cm;        the voltage source comprises a capacitor with a capacitance above 1 μF, preferably above 10 μF;        the capacitance of the capacitor is adjustable, preferably below 1000 μF, more preferably below 200 μF;        the circuit further comprises a Marx generator and ferrites forming a saturable inductance in a path leading the capacitor directly to the inductance, the ferrites being saturated once the Marx generator has discharged;        the capacitor is separated from the inductance by a spark-gap that can be triggered by a pulse generator;        the voltage source comprises a Marx generator (118), said Marx generator preferably having adjustable characteristics;        the electrodes have a radius between 0.1 mm and 50 mm, preferably between 1 mm and 30 mm;        the device is mobile and is fixed before generating an electric arc;        the device comprises an uncoupling system;        the device comprises several pairs of electrodes.        
A method is also proposed for fracturing a geological hydrocarbon reservoir, in which said method comprises electrical fracturing of the reservoir by generating an electric arc in a fluid present in a well drilled in the reservoir, the electric arc inducing a pressure wave the rise time of which is greater than 0.1 μs, preferably greater than 10 μs. According to examples, the method can comprise one or more of the following features:                the arc is generated by the device described above;        the voltage source is charged by a high-voltage charger to a voltage between 1 and 500 kV, preferably between 50 and 200 kV;        the method further comprises static fracturing of the reservoir by hydraulic pressure, preferably the static fracturing precedes the electrical fracturing;        the well is horizontal;        electrical fracturing is repeated in various treatment zones along the well and/or in which several arcs are generated in succession in each treatment zone.A method is also proposed for the production of hydrocarbons comprising the fracturing of a geological hydrocarbon reservoir by the method described above.        